At the time of writing, 1 BTC = ~ 200 USD, and is holding fairly steadily there.
Thus 1 USD = ~ .005 BTC, which is kind of an ugly little conversion, and it requires more of a cognitive leap from using the currency people are used to, to using this new currency.
Fortunately, we have the mBTC, which is not shockingly worth 0.001 BTC.
At the time of writing, 5 mBTC = ~ 1 USD, which is a much nicer conversion. You don’t need to count zeroes nearly as much.
I also like that the mBTC gives businesses listing their prices in bitcoin a new opportunity to use the classic 99 cents ending that is so ubiquitous in dollarslandia, that we don’t even need to think about it.
How much is that t-shirt? Oh, it’s 7.99 mBTC. Looks nice, doesn’t it?
Bitcoins are too expensive
Another good reason to start using mBTC, is that $200 for a bitcoin sounds really expensive! $0.20 for an mBTC is much more reasonable.
Is it rational, no. But then humans aren’t always rational either.
(credit for the mBTC logo at the top of this post goes to http://www.em-bit.org).